The present invention relates to apparatus for and methods of limiting liquid outlet pressures developed by reciprocating pumps, and in particular to pressure responsive drivers for double-acting airless pumps.
In airless spray painting, paint is supplied by a pump at high pressures on the order of about 1000 to 3000 psi to an atomizing nozzle having a small elliptically shaped orifice, and is hydraulically atomized into a fan-shaped spray upon exiting the orifice. It is conventional to spray intermittently, and a valve associated with the nozzle accommodates starting and stopping of the spray.
Some means must be provided to control internal paint pressure within the pump to prevent building up excessive pressure, particularly when the spray nozzle valve is closed. It has been proposed to use air or pneumatic motors to actuate the pump, such motors being operable only when required to meet a pressure requirement in the pump. Air motors, however, have not fulfilled all of the requirements of the art, since they require a source of air, which in the case of allegedly portable equipment requires bulky and expensive air compressors and pressure tanks.
Electric motors and gasoline engines have also been used to drive high pressure pumps for airless spraying. Most, if not all, of such units have some means for controlling the maximum liquid pressure developed by the pump, often to between 2500-3000 psi. A common type of pressure control uses a bourdon tube to sense liquid pressure and actuate a microswitch in response to the pressure reaching a selected maximum value. In the case of an electric motor driven pump, to limit pump pressure the motor may be turned on and off, often as many as 40 to 50 times per minute. On the other hand, for either electric motor or gasoline engine driven pumps, pressure control may be accomplished through an electric clutch that engages and disengages the continuously running motor or engine with and from the pump, again at approximately 40 to 50 times per minute. These pressure controls use an assortment of electronic parts such as microswitches, triacs, solid state motor starters, motor start capacitors, etc., which must be enclosed in explosion proof housings. Such electronic parts represent a principal cost component of airless pumps.
The electronic components of conventional pressure controls also are a major source of field maintenance problems. Excessive cycling of a capacitor start electric motor burns out the contacts for the starting capacitor, and because of the high surge current when the motor is started, average current draw is high and results in a shortened life of the motor, starting capacitor and start switches. Also, bourdon have a finite life and can rupture from airless pressures. Should a bourdon tube rupture, paint filling the control box not only will destroy the controls, but in the case of a clutch drive it can then flow through the control lead conduit to the clutch and destroy it. Further, the high surge current occurring at 50 to 60 times per minute pits the start switch contacts and can cause them to fuse together and keep the motor running or clutch engaged, resulting in excessive pressure at the pump outlet and rupture of the bourdon tube.